Super Bowl replays are available for viewing at NFL.com. Video of past Super Bowls is often available on sites such as YouTube as well, but the quality can be lower. New sites such as ABC News sometimes have recent Super Bowl replays.
Older Super Bowl replays may only be available by purchasing video. Some fans trade videos with one another, but since these videos are copyrighted, it is potentially illegal for fans do this. Generally, the older the game, the harder it is to find replays. Online auctions sites such as eBay.com can have videos for sale. Rarer game footage is considered highly collectible and can be expensive.